The invention is an improved ball check valve, for use particularly in chemical metering pumps. Such pumps deliver fluid against pressure differentials ranging from almost zero to about 300 psi; the pumped fluid may be corrosive, solvent or oxidizing. Such pumps typically operate in manufacturing environments where they may be subject to impact; the valves must operate reliably over a large number of cycles, typically tens of millions. The valves are generally the first elements of such a pump to wear, and therefore must be designed to be both durable in use and relatively easy to replace in the field, by mechanics or other personnel who may be unfamiliar with the valve construction.
Many designs of such valves have been used; the present invention relates to a ball check valve. A ball check valve basically comprises a fluid port of circular cross section and a spherical closure element or ball which in a first direction of fluid flow (as during the pressure stroke) floats away from the port, permitting flow, and in a second direction of flow (as during the suction stroke) seats in the port, stopping flow. The port and ball are contained within a valve body, which generally comprises at least two parts which are secured together as by mating screw threads, permitting assembly of the valve or replacement as needed.
In such a valve two fluid paths must be interrupted by seals: first, the path of desired flow of the fluid being pumped (fluid path) which is alternately sealed and unsealed during the pump operating cycle; second, the path of leakage from the fluid path through the interface between the valve body parts (leakage path) which is desirably sealed at all times. The fluid path is sealed by the ball and port; the leakage path is primarily sealed by mutually tightening the two valve body parts; ancillary sealing means such as an O ring may be used to improve this seal.
In practice various difficulties are found with such valves. To withstand the forces exerted over many cycles of operation, and also to resist the destructive action of the pumped fluid, the ball and port are typically made of hard substances such as metal or ceramic, and unless finished, for example by machining, to extremely close tolerances will not seal completely to one another. Such precise finishing adds to the manufacturing cost of the valve. To avoid the need for such finishing, therefore, an additional resilient seat element such as an elastomeric O ring is typically employed between the port and the ball to provide a resilient valve seat, which accommodates to irregularities in the ball or in the port surface on which it is supported, as well as to any misalignment between ball and port.
A problem with the use of such a resilient seat element has been that when compressed against a supporting surface that is not adequately flat, the seat element, being elastomeric, tends to conform with any excursions in the surface and therefore is distorted and fails to seal completely against the ball. The supporting surface is therefore desirably provided by a relatively hard element; such hard material is often finished to provide the accurate supporting surface, but such finishing is a relatively expensive method of manufacture. Moreover there are problems of sealing the leakage path past such element.
A further problem with the use of such seat elements has been that when the pump operates across a low pressure differential, at the end of the pressure stroke there is only a small force urging the ball to seat in the port. It is therefore particularly important that the ball seat accurately and completely against the resilient seat element even under low pressure. However the ball, urged toward the valve seat by the back pressure of the fluid, does not move along a well defined path; hence the position at which it first contacts the seat element is not predictable. Sealing may not be rapid, permitting excessive back flow or "slippage", and complete sealing may not even be attained.
In some ball check valve applications, springs have been employed to assist the seating of the ball; however, because of the corrosive nature of the fluid pumped by chemical metering pumps, such springs are generally unsatisfactory in this environment. A resistant coating on the spring not merely increases its cost, but also tends to make the spring stiffer, bulkier and less functional. Therefore some other means must be found to provide for rapid and accurate seating of the ball even at very low pressure. At the same time, the valve design must permit the ball to move sufficiently away from the port to provide adequate flow area.
Undesirable distortion of the seal element may also result from axial and tangential forces exerted as the housing parts are mutually tightened. Such a distortion tends to degrade the function of the seal element.
It is therefore desirable to provide a ball check valve assembly suitable for use in the demanding chemical metering pump environment, in which the movable closure element reliably seats itself rapidly and accurately against the resilient seat element over a range of differential pressures from zero or almost zero through at least 300 psi, while at the same time the leakage path is reliably sealed off. It is an object of the present invention to provide such a ball check valve assembly which is inexpensive to manufacture, has relatively few and simple parts, and is easily positioned for use or removed and replaced.